This invention relates to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
The twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into smaller vessels before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the production of cast strip.
During casting, the casting rolls rotate such that metal from the casting pool solidifies into shells on the casting rolls that are brought together at the nip to produce a cast strip downwardly from the nip. One of the difficulties in the past has been high frequency chatter, which should be avoided because of surface defects caused in the strip. Temperature increase as the cast strip leaves the nip, called temperature rebound, is also a concern, and can cause enlargement of the shell due to ferrostatic pressure from the casting pool resulting in ridges in the strip. Temperature rebound occurs when the center of the strip contains “mushy” material, i.e. the metal between the shells that have not solidified to be self supporting, and the latent heat from the center material will cause the shells to reheat after leaving the casting rolls.
We have found that the defects caused by high frequency chatter and temperature rebound can be controlled by maintaining and controlling the amount of mushy material that is “swallowed” in the cast strip and subsequently cooled. Some mushy material sandwiched between the solidified shells is provided to cushion the unevenness in the growth and cooling of the shells and inhibits if not eliminates high frequency chatter and the attendant strip defects. At the same time, the amount of mushy metal between the solidified shells is controlled to reduce and control the amount of temperature rebound in the cast strip. If the rebound temperature is not controlled, it can cause at least partial remelting of the solidified shells and defects in the strip such as ridges, and in severe circumstances, breakage of the strip where the temperature is too high and more excessive remelting of the shells occur. The mushy material may include molten metal and partially solidified metal, and includes all the material between the shells not sufficiently solidified to be self supporting.
To further explain, immediately below the nip the mushy material in the strip is in communication with the casting pool due to the ferrostatic pressure. When an excess amount of mushy metal is between the shells of the strip below the nip, a high temperature rebound begins to re-melt and weaken the solidified shells of the cast strip. Weakened shells may locally bulge due to the ferrostatic pressure causing local excessive strip budge, surface defects in the cast strip, and severe weakening may cause strip breakage. Also, when an excess amount of mushy material is between the shells near the strip edges, the mushy material may enlarge the edges of the strip causing “edge bulge,” or may drip from the edges of the cast strip causing “edge loss.”
We have found desired properties by maintaining a consistent austenitic microstructure in the cast strip at the hot rolling mill downstream of the caster. The increased temperature from temperature rebound may re-heat the strip to a temperature forming δ-ferrite, which upon cooling returns to a coarser and more variable austenite microstructure.
We presently disclose a method where temperature rebound and its attendant strip defects can be controlled while inhibiting high frequency chatter. Disclosed is a method of continuously casting metal strip including                assembling a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a gap at a nip between the casting rolls through which thin cast strip can be cast,        assembling a metal delivery system adapted to deliver molten metal above the nip to form a casting pool supported on the casting surfaces of the casting rolls and confined at the ends of the casting rolls and counter rotating the casting rolls to form metal shells on the casting surfaces of the casting rolls that are brought together at the nip to deliver cast strip downwardly with a controlled amount of mushy material between the metal shells,        determining at a reference location downstream from the nip a target temperature for the cast strip corresponding to a desired amount of mushy material between the metal shells of the cast strip,        sensing the temperature of the cast strip cast downstream from the nip at the reference location and producing a sensor signal corresponding to the sensed temperature, and        causing an actuator to vary the gap at the nip between the casting rolls in response to the sensor signal received from the sensor and processed to determine the temperature difference between the sensed temperature and the target temperature.        
The gap between the casting rolls may be varied by the actuator to control the amount of mushy material between the metal shells of the strip cast to be between about 10 and 200 micrometers in response to the processed sensor signal. Alternatively, the amount of mushy material between the metal shells of the strip cast may be between about 10 and 100 micrometers in response to the processed sensor signal. In yet another alternative, the amount of mushy material between the metal shells of the strip cast may be between about 20 and 50 micrometers in response to the processed sensor signal.
The casting rolls may be counter-rotated to provide a casting speed between about 40 and 100 meters per minute, and the as-cast thickness of the cast strip may be between about 0.6 and 2.4 millimeters.
The casting pool height may be between about 125 and 250 millimeters above the nip. The heat flux density through the casting rolls may be between about 7 and 15 megawatts per square meter of casting roll surface.
An apparatus for continuously casting metal strip may include                a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a gap at a nip between the casting rolls through which thin cast strip can be cast,        a metal delivery system adapted to deliver molten metal above the nip to form a casting pool supported on the casting surfaces of the casting rolls and confined at the ends of the casting rolls that are brought together at the nip to deliver cast strip downwardly from the nip with a controlled amount of mushy material between the metal shells,        a sensor adapted to sensing the temperature of the cast strip downstream from the nip at a reference location and producing a sensor signal corresponding to the temperature of the cast strip below the nip, and        a controller adapted to control an actuator to vary the gap between the casting rolls to provide a controlled amount of mushy material between the metal shells of the cast strip at the nip in response to the sensor signal received from the sensor and processed to determine the temperature difference between the sensed temperature and a target temperature.        
Again, the gap between the casting rolls may be varied by the actuator to control the amount of mushy material between the metal shells of the strip cast to be between about 10 and 200 micrometers in response to the processed sensor signal. Alternatively, the amount of mushy material between the metal shells of the strip cast may be between about 10 and 100 micrometers in response to the processed sensor signal. In yet another alternative, the amount of mushy material between the metal shells of the strip cast may be between about 20 and 50 micrometers in response to the processed sensor signal.
Again, the casting rolls may be counter-rotated to provide a casting speed between about 40 and 100 meters per minute, and the as-cast thickness of the cast strip may be between about 0.6 and 2.4 millimeters.
Again, the casting pool height may be between about 125 and 250 millimeters above the nip. The heat flux density through the casting rolls may be between about 7 and 15 megawatts per square meter of casting roll surface.
One or more sensors are provided adapted to sensing the location of the casting rolls and producing a sensor signal corresponding to the position of the casting rolls. Alternatively or in addition, one or more sensors may be provided adapted to sensing a force exerted on the cast strip adjacent the nip and producing a sensor signal corresponding to the force exerted on the cast strip adjacent the nip.
Also disclosed is a method of continuously casting metal strip including the steps of:                assembling a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a gap at a nip between the casting rolls through which thin cast strip can be cast,        assembling a metal delivery system adapted to deliver molten metal above the nip to form a casting pool supported on the casting surfaces of the casting rolls and confined at the ends of the casting rolls and counter rotating the casting rolls to form metal shells on the casting surfaces of the casting rolls that are brought together at the nip to deliver cast strip downwardly with a controlled amount of mushy material between the metal shells,        determining at a reference location downstream a target temperature for the cast strip from the nip corresponding to a desired amount of mushy material between the metal shells of the cast strip to produce a desired strip crown,        sensing the temperature of the cast strip cast downstream from the nip at the reference location and producing a sensor signal corresponding to the sensed temperature, and        causing an actuator to vary the gap at the nip between the casting rolls in response to the sensor signal received from the sensor and processed to determine the temperature difference between the sensed temperature and the target temperature to produce the desired strip crown.        
The step of determining a target temperature may include the steps of receiving a customer-specified strip crown, and determining the target temperature to produce the customer-specified strip crown.